However, there are some product compromises that would preclude me from using this. First, I already manage drive purchases, parity, dealing with scrubs and errors, etc. for my pool. I don't want to duplicate my effort for a second backup pool. ZFS sends are, ultimately just a long binary, and I simply want them stored, without much hassle. If I'm planning a zpool expansion, the work in using this service is now doubled, having to (non-trivially) determine the best pool geometry for my own pool, and for a backup pool made up of differently sized drives.
I also don't understand why someone would want to give you their encryption passphrase. A service like this should just take encrypted sends, and send them back when requested. I don't want my plaintext anywhere other than my own hardware.
That said, your main competitor suffers from all of these problems as well.